Conventionally, fuel cell co-generation systems with high power generation efficiency and high overall efficiency (hereinafter, simply referred to as “fuel cell systems”) have been drawing attention as distributed power generators that make efficient energy utilization possible.
A fuel cell system includes a fuel cell as the main body of its power generating part. Examples of the fuel cell include a phosphoric acid fuel cell, a molten carbonate fuel cell, an alkaline fuel cell, a polymer electrolyte fuel cell, and a solid oxide fuel cell. Among these fuel cells, the operating temperature of a phosphoric acid fuel cell or a polymer electrolyte fuel cell (abbreviated as “PEFC”) during a power generation operation is relatively low. Therefore, these fuel cells are suitably used for forming fuel cell systems. In particular, electrocatalyst degradation of a polymer electrolyte fuel cell is less than that of a phosphoric acid fuel cell. In addition, electrolyte dissipation does not occur in a polymer electrolyte fuel cell. Therefore, in particular, polymer electrolyte fuel cells are suitably applied in handheld electronic devices and electric automobiles, for example.
In most fuel cells, for example, in a phosphoric acid fuel cell or a polymer electrolyte fuel cell, hydrogen is used as a fuel in a power generation operation. However, usually, means for supplying hydrogen necessary in the power generation operation of such a fuel cell is not developed as an infrastructure. For this reason, in order to obtain electric power by means of a fuel cell system including a phosphoric acid fuel cell or a polymer electrolyte fuel cell, it is necessary to generate hydrogen as a fuel at the installation location of the fuel cell system. Therefore, in conventional fuel cell systems, it is often the case that a fuel cell is installed together with a hydrogen generation apparatus. In the hydrogen generation apparatus, a hydrogen generation method, for example, a steam reforming method, is used to generate a hydrogen-containing gas. In the steam reforming method, a hydrocarbon-based raw material such as natural gas, propane gas, naphtha, gasoline, or kerosene (i.e., a raw material gas) is mixed with water, or alternatively, an alcohol-based raw material such as methanol is mixed with water. The mixture is supplied to a reformer including a reforming catalyst. In the reformer, a steam reforming reaction progresses, and thereby a hydrogen-containing gas is generated.
The hydrogen-containing gas, which is generated by the reformer of the hydrogen generation apparatus with the steam reforming method, contains carbon monoxide (CO) generated as a by-product. For example, the hydrogen-containing gas generated by the reformer of the hydrogen generation apparatus contains carbon monoxide in a concentration of about 10 to 15%.
The carbon monoxide contained in the hydrogen-containing gas significantly poisons the electrocatalyst of a polymer electrolyte fuel cell. The poisoning of the electrocatalyst causes significant degradation in the power generation performance of the polymer electrolyte fuel cell. Therefore, in conventional hydrogen generation apparatuses, it is often the case that the reformer generating the hydrogen-containing gas is installed together with a CO reducer in order to sufficiently reduce the carbon monoxide concentration in the hydrogen-containing gas. The CO reducer reduces the carbon monoxide concentration in the hydrogen-containing gas generated by the reformer to 100 ppm or lower, and preferably, 10 ppm or lower. The hydrogen-containing gas from which carbon monoxide has sufficiently been removed is supplied to the fuel cell of the fuel cell system during a power generation operation. In this manner, poisoning of the electrocatalyst in the polymer electrolyte fuel cell can be prevented.
It should be noted that, usually, the CO reducer included in the hydrogen generation apparatus includes a shift converter. The shift converter is configured to cause a water gas shift reaction catalyzed by a shift conversion catalyst disposed inside the shift converter, thereby generating hydrogen and carbon dioxide from carbon monoxide and steam. The CO reducer further includes a purifier positioned downstream from the shift converter, the purifier including at least one of an oxidation catalyst and a methanation catalyst. The oxidation catalyst causes an oxidation reaction between oxygen in air and carbon monoxide to progress. The methanation catalyst causes a methanation reaction of carbon monoxide to progress. By means of the shift converter and the purifier, the carbon monoxide concentration in the hydrogen-containing gas generated by the reformer is reduced to 100 ppm or lower.
Natural gas supplied to the reformer of the hydrogen generation apparatus as the raw material usually contains a trace amount of nitrogen. The nitrogen content in the natural gas varies depending on, for example, the area where the natural gas is supplied. During a power generation operation of the fuel cell system, there is a case where when the natural gas containing nitrogen is supplied to the reformer of the hydrogen generation apparatus, a chemical reaction between hydrogen generated through a steam reforming reaction and nitrogen is catalyzed by a reforming catalyst included in the reformer, and thereby ammonia is generated. Ammonia is a chemical agent that causes significant degradation in the power generation performance of the polymer electrolyte fuel cell. In addition, ammonia may poison the oxidation catalyst provided in the purifier depending on the type of the catalyst. The poisoning of the oxidation catalyst by ammonia causes significant degradation in the carbon monoxide removing performance of the purifier. This causes poisoning, by carbon monoxide, of the electrocatalyst in the polymer electrolyte fuel cell. Here, the poisoning of the electrocatalyst by carbon monoxide causes more significant degradation in the power generation performance of the polymer electrolyte fuel cell as compared to the power generation performance degradation caused by ammonia. Therefore, removing ammonia from the hydrogen-containing gas by means of an ammonia remover before supplying the hydrogen-containing gas to the polymer electrolyte fuel cell is not enough to stably obtain electric power from the fuel cell system, and it is necessary to suppress the significant power generation performance degradation that is caused by the poisoning of the oxidation catalyst by ammonia.
In this respect, there is a proposed hydrogen generation apparatus that is configured to perform a regeneration operation in accordance with the progress of oxidation catalyst degradation caused by ammonia (see Patent Literature 1, for example).